This application is related to the following previously-filed U.S. patent applications: Ser. No. 08/248,176, filed May 24, 1994, Ser. No. 08/253,283, filed Jun. 2, 1994, Ser. No. 08/327,370, filed Oct. 21, 1994, and Ser. No. 08/358,880, filed Dec. 19, 1994, all assigned to the same assignee as the present application and all of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to recording and playback of digital video signals.
2. Background of the Invention
Television signals have traditionally been transmitted and recorded in analog form, but more recently digital transmission and recording have been introduced. Digital video cassette recorders (VCRs) have been introduced to the market, and digital broadcast of television signals has been introduced to the U.S. in the form of direct satellite-to-home digital television systems.
Digital broadcast of television signals has permitted the introduction of advanced digital signal compression and encoding techniques, reducing the bandwidth of television signals without loss of picture quality. These techniques include the Moving Picture Expert Group (MPEG) transmission standard, ISO/IEC 13818-1, in which the video signal includes so-called I-frames, which have a large quantity of data representing a complete still image, and sequences of so-called P-frames and B-frames, which have a smaller amount of data representing incremental modifications of the still image which produce motion. I-frames are transmitted at a low rate interspersed with a number of P-frames and B-frames.
It has been proposed, in the above-referenced related U.S. Patent Applications, to record digital television broadcast signals using a digital VCR, to thereby maintain fidelity of the broadcast signal during recording and subsequent playback.
This use of the digital VCR raises difficulties arising from the data compression techniques often applied to digital broadcast signals. These digital data compression techniques typically involve variable-length encoding in which each data value is generated as a function of possibly several prior data values, and thus an error in reproduction of a given data value can produce a lengthy string of consecutive data errors.
To prevent such lengthy strings of errors from impacting reproduction quality, it has been proposed in above-referenced application Ser. No. 08/358,880 to include three levels of redundancy, or parity, encoding into the data recorded by the digital VCR. The first, second and third levels of parity encode increasingly larger segments of data, such that the third, highest level of parity encodes roughly 100 kilobytes of data. The very large amount of data encoded by the third level of parity provides protection from lengthy data errors of the kind that may be produced by run-length encoding such as is typically used in digital video signals.
The use of multiple levels of parity makes it difficult to encode and decode data, since very large quantities of data must be handled and manipulated in a relatively short period of time during both recording and playback. Moreover, since digital memory is relatively expensive, it is important to perform the encoding and decoding operations using a small amount of digital memory.
The present invention provides a method for managing parity encoding and decoding in a manner which provides efficient use of digital memory, and further provides a digital video recorder and/or player which utilizes this method.
specifically, in accordance with principles of the present invention, recording is a two-phase operation. In a first phase, a first portion of a digital memory is used to output error-encoded data and to store new digital data from the signal source. After an error-encoded digital signal in the first portion of the memory has been forwarded to the recording head, digital signals from the signal source are stored in its place in the first portion of the digital memory. At the same time, during the first phase, error correcting circuitry error encodes digital signals stored in a second portion of the digital memory. In the second phase of the recording operation, the roles of the memory portions are reversed: error-encoded digital signals in the second portion of the memory are forwarded to the recording head, and digital signals from the signal source are stored in place of these error-encoded digital signals in the second portion of the digital memory. At the same time, the error correcting circuitry error encodes digital signals previously stored in the first portion of the digital memory.
Playback is also a two-phase operation. In the first playback phase, a first portion of a digital memory is used to output error-decoded data to a destination such as a television, and to store new digital data from the playback head. After an error-decoded digital signal in the first portion of the memory has been forwarded to the destination, digital signals from the playback head are stored in its place in the first portion of the digital memory. At the same time, during the first phase, error correcting circuitry error decodes digital signals stored in a second portion of the digital memory. In the second phase of the recording operation, the roles of the memory portions are reversed: error-decoded digital signals in the second portion of the memory are forwarded to the destination, and digital signals from the signal source are stored in place of these error-decoded digital signals in the second portion of the digital memory. At the same time, the error correcting circuitry error decodes digital signals previously stored in the first portion of the digital memory.
The two memories used in the above-noted phases may be implemented as separate integrated circuits, or may be paged into a single integrated circuit memory chip.
Apparatus in accordance with the invention may include either or both of the record and playback functions described above, and the specific embodiment of the invention described below performs both functions.
In this specific embodiment, in both record and playback modes, there is a transition period between the two phases described above; this transition period occurs after completion of most or all of the error-encoding or error-correcting, and before the memories switch roles and error-encoding or error-correcting begins in a new memory. For example, during the transition period between the first and second phases of a playback operation, error-corrected digital signals in the second memory, which are the result of the recently-completed error correcting operation, are read from the second memory and sent to the signal destination; at the same time, digital signals from the playback head continue to be written into the first memory. This transitional period continues until the second memory is full, at which point the second phase, as described above, begins. As a second example, during the transition period from the first to the second phase of a record operation, error-encoded digital signals are read from the second memory and sent to the record head while digital signals from the signal source are still being written into the first memory.
In the specific embodiment discussed below, recording and playback are performed by a video tape system using a rotating drum. In other embodiments, the recording and playback may be performed by a video disk rotating under a radially-positioned head, or by any other suitable digital recording media. The video tape system described below permits variable-speed reproduction of previously recorded data, by reconstructing tracks of data on the video tape from multiple passes of the playback head.
Furthermore, in the specific embodiment discussed below, the encoded signal uses a compressed format such as the MPEG format discussed above. A packet detector for identifies those MPEG frames including complete still images, so that these still images may be used to generate trick play data that can be recorded along with the MPEG data to permit special effect playback functions. This trick play data is reconstructed upon playback and used in special effects playback modes. To do so, the playback apparatus includes an encoder for forming MPEG-compatible packets from the trick play data, which can be forwarded for display while in a special effects playback mode.